Although you can compile VirtualBox like Ralph Ellis did, the fastest and easiest way to have a working VirtualBox setup on a PCBSD host is to download and install the VirtualBox PBI from this location:
After starting the VBox machine, create a "new" guest by configuring the type, disk size, memory size, video settings, etc. Put your install media (i.e. your Windows installation CD) into the CD drawer and start the new guest to install the guest operating system. Don't forget to install "Guest Additions" to enhance the use of the guest operating system.